The present invention relates to detecting the unauthorized removal (tamper) of a wearable transmitter tag. More particularly, the present invention relates to a tamper detection circuit and method for detecting the unauthorized removal of a transmitter tag used in an electronic house arrest monitoring (EHAM) system, or equivalent system designed to monitor ambulatory objects or persons.
EHAM systems are known in the art. See, e.g., U.S. Pat. Nos. 4,885,571; 4,918,432 and 4,952,913, issued to Pauley et al., all of which are incorporated herein by reference. As indicated in those references, house arrest (a court-ordered mandate that requires a convicted law breaker to remain at a specific location, e.g., his or her house, at specified times) represents a very significant and viable alternative to conventional incarceration of convicted law breakers, especially those found guilty of non-violent crimes.
A typical EHAM system includes a transmitter tag that is securely attached to a limb of the person being monitored, and a field receiver that is mounted within the location whereat the offender is to remain. The transmitter tag periodically transmits an identifying signal that uniquely identifies its wearer. If the offender is within range of the field receiver, i.e., at the designated house arrest location, the field receiver receives and logs the identifying signal. If the offender is not within range of the field receiver, i.e., not at the designated house arrest location, the field receiver notes the absence of the identifying signal. Periodically or as needed, telecommunicative contact is established between the field receiver and a central monitoring computer so that the information received by the field receiver can be downloaded to the central computer.
While those sentenced to house arrest (hereafter the "offender") will generally recognize the need and benefit of complying with the sentence imposed, there nonetheless remains the need to monitor the presence or absence of the offender to ensure that the sentence imposed is being followed. Disadvantageously, the offender may sometimes attempt to foil the monitoring system by removing the transmitter tag. Hence, there is a further need in the house arrest monitoring art to detect any attempts by the offender to remove the transmitter tag, so that such events (hereafter "tamper events") can be promptly reported to the central monitoring computer.
One type of transmitter tag that has been used in EHAM systems of the prior art is essentially a two-piece molded structure inside of which the electronic circuits and batteries of the transmitter circuits are placed. Once the electronic circuits and batteries are placed inside of the unit, the two pieces of the case are permanently bonded or glued to each other, thereby creating a unitary construction. Unfortunately, such unitary sealed transmitter tag is useful only for the life of the battery, and then the tag must be discarded.
Another type of tag is disclosed in U.S. Pat. No. 4,812,823 issued to Dickerson, incorporated herein by reference. Dickerson teaches a tag case having a removable battery pack assembly that can be lockably secured to the tag case. As disclosed in Dickerson, an important feature of a portable tag used in personnel monitoring is that a strap that closes the tag around the wearer of the tag should be tamper resistant. One way of making such straps tamper resistant is to include a tamper detection circuit within the tag that detects an attempt to cut or otherwise violate the strap. Advantageously, such tamper detection circuit not only provides a means of notifying an operator at a remote location that the wearer has violated the strap, but also provides a substantial psychological deterrent to such violations. Dickerson teaches the use of a conductive material for the strap, thereby allowing anti-tamper electrical circuits within the tag to periodically perform electrical continuity checks to verify that the strap has not been cut.
Problematically, however, the Dickerson tamper detection means may be foiled by the wearer of the tag in at least two ways. First, the wearer may attach a parallel electric current path to opposite ends of the strap, e.g., by using a jumper cable having an alligator clip at both ends. Having attached the parallel electric current path to the strap, the strap can then be cut near the middle of the strap without breaking the electrical continuity of the anti-tamper electrical circuits. Thus, the wearer of the tag may easily defeat the Dickerson tag's anti-tamper circuits.
Second, if the Dickerson tag is immersed in an electrolyte solution and then the strap is cut, the electrolyte solution serves as a parallel electric current path to the strap. The strap can thus be cut without breaking the electrical continuity of the antitamper electrical circuits.
One possible way to detect tampering with the strap that cannot be easily foiled by using a parallel electric current path (a jumper) is through the use of a capacitance detector consisting of at least two electrodes. Such a capacitance detector is shown in U.S. Pat. No. 4,885,571, issued to Pauley et al., previously incorporated herein by reference. In '571 Pauley, a continuity check of a conductive strap or band that holds the tag on the wearer is combined with a capacitance detector. The capacitance detector comprises the strap or band (a first electrode) and a capacitor plate (a second electrode). A capacitance detection circuit is used to detect a change in the capacitance between the two electrodes. Normally, when the tag is worn, the strap wraps around a limb of the offender, e.g., around the offender's ankle. The capacitor plate, being housed within the tag case, is coupled to the strap via electrostatic coupling though the body mass around which the strap is closed. In the event the strap is cut after establishing a parallel electrical path using a jumper cable or equivalent, the capacitance detector detects a change in the capacitance between the two electrodes. The change in capacitance is due to the fact that the body mass around which the strap was closed is absent, thereby causing the electrostatic coupling to occur through a different medium, e.g., air, instead of through the body mass. Such different coupling causes the dielectric material of the capacitor to change; and, as a result, causes the capacitance of the capacitor to change. Thus, such detectable change in capacitance is used to indicate that the strap has been violated.
Problematically, however, the '571 Pauley tag can still be foiled by first immersing the tag in an electrolyte before cutting the strap. For example, saltwater, which is a good conductor and will serve as a parallel electrical path, has approximately the same dielectric characteristics, i.e., permittivity, as the body mass, i.e., flesh. Thus, by immersing the Pauley tag in an electrolyte, the anti-tamper circuits can be foiled.
What is thus needed is a way to detect violations of the strap of a wearable transmitter tag that cannot be foiled by either (1) creating an electric current path parallel to the strap before violating the strap; or (2) immersing the tag and strap in a suitable electrolyte before violating the strap.
The present invention advantageously addresses the above and other needs.